Various specialized cooling applications require refrigeration systems with very low capacities. A system having a capacity in the range of one-tenth ton (350 watts) which uses common refrigerants such as R12, R134A, R22 or R502, requires a thermostatic expansion valve that regulates refrigerant flow within a range of very low flow rates.
Conventional thermostatic expansion valves designed for use in much larger systems, use a single needle/ball and orifice combination to regulate refrigerant flow. The proximity of the needle to the orifice is controlled to regulate the amount of refrigerant that passes through the orifice. The flow area of the orifice in a refrigeration system having a capacity of one-fourth ton (900 watts) or less, is extremely small. It is not feasible to economically build an expansion valve for such a small system that uses a single needle/ball and orifice construction.
One approach that has been taken to produce a thermostatic expansion valve for a system which operates at a low refrigerant flow rate is to provide a valve in which the refrigerant undergoes double expansion. In valves of this type, the refrigerant is first expanded through a conventional needle/orifice. Thereafter the refrigerant is passed through a second, extremely small, metering orifice. Such a valve has been previously produced by the Danfoss Company. The drawback of this prior art construction is that it is difficult to achieve a precisely sized metering orifice having an area necessary to achieve the desired flow range. This makes the valve difficult to produce, imprecise in controlling refrigerant flow and expensive.
Thus, there exists a need for a thermostatic expansion valve that may be used in low capacity mechanical refrigeration systems, that precisely controls refrigerant flow and which is less expensive to produce.